import sys
from PyQt5.QtCore import *
from PyQt5.QtGui import *
from PyQt5.QtWidgets import *


class Dialogdemo(QMainWindow):
    def __init__(self, parent=None):
        super(Dialogdemo, self).__init__(parent)
        self.setWindowTitle('showDialog')
        self.resize(350, 300)

        self.btn = QPushButton(self)
        self.btn.setText('show')
        self.btn.move(50, 50)
        self.btn.clicked.connect(self.showdialog)
    def showdialog(self):
        dialog = QDialog()
        btn = QPushButton('ok', dialog)
        btn.move(50, 50)
        dialog.setWindowTitle('son dialog')
        dialog.setWindowModality(Qt.ApplicationModal)
        dialog.exec_()


if __name__ == '__main__':
    app = QApplication(sys.argv)
    de = Dialogdemo()
    de.show()
    sys.exit(app.exec_())
